1. Field of the Invention
The present invention provides a lens set for a projection apparatus. In particular, the present invention provides a lens set for reducing light loss in a projection apparatus.
2. Descriptions of the Related Art
Due to advantages, such as a long service, life, low operation temperature and immediate switching capabilities, light emitting diodes (LEDs) have gradually replaced ultra high performance (UHP) mercury bulbs as light sources of projection apparatuses. When LEDs are adopted as light sources of the projection apparatus, the design and disposition of optical elements in the projection apparatus must also be considered to achieve desirable light availability and light distribution uniformity.
With regards to light availability, most LEDs are formed into the shape of a square so that the shape of the light projection emitted by an LED is also substantially square-shaped. However, when a square light is projected onto a non-square micro-display device such as a digital micromirror device (DMD), a liquid crystal display (LCD), or a liquid crystal on silicon (LCOS) (because the micro-display device must be of a specific length-to-width ratio to fit display requirements), the shape of the light fails to conform to that of the micro-display device, thereby leading to reduction in the light availability. For purposes of minimizing the light loss when the light is projected onto the micro-display device, a number of optical elements for altering the shapes of light projection have emerged. Common optical elements include tapered light pipes, anamorphic prisms and cylinder lenses. By means of such optical elements, the light projection shape of the LED can be altered to satisfy an aspect ratio when being projected onto the micro-display device; however, when the light passes through the aforesaid optical elements, the light source shape tends to be altered, leading to problems such as a mismatch between the light source shape with the projection lens opening.
Furthermore, when a single lens is used to focus, light distribution non-uniformity can occur when the light is projected onto the micro-display device through the single lens. The non-uniformity results in too much luminance on the central portion of the resulting image but insufficient luminance on the peripheral portions. Accordingly, a lens array has been developed in the art to overcome these shortcomings.
FIG. 1A illustrates a projection apparatus 1. The projection apparatus 1 comprises an LED light source module 11, lens arrays 13, 14 and a micro-display device 15. The LED light source module 11 has a plurality of LEDs 12. Light generated by the LEDs 12 is focused by a plurality of lens units 131, 141 of the two lens arrays 13, 14 in such a way that the light is divided into a plurality of blocks for projection, thereby mitigating the over-concentration of the light when focused using a single lens. Thus, uniform distribution of the light on the micro-display device 15 can be achieved.
With reference to FIG. 1B and FIG. 1C together, an optical path will be described in detail. FIG. 1B illustrates a schematic view of a first optical path. Here, the lens array 13 focuses the light of the LED light source module 11 onto the lens units 141 of the lens array 14. To increase the light availability, the length-to-width ratio of each of the lens units 141 is made to be identical to that of the LED light source module 11. In other words, in order for the light to be utilized completely with no light leakages after the light of the LED light source module 11 is focused, the length-to-width ratio of each of the lens units 141 is identical to that of the LEDs of the LED light source module 11. Next, FIG. 1C illustrates a schematic view of a second optical path. In the second optical path, because the light transmitted through each of the lens units 131 must be focused onto the micro-display device 15 through the lens array 14, the lens units 131 and 141 must be designed to have the same area in order for the light of each of the lens units 131 to be focused onto the micro-display device 15 through a corresponding lens unit 141. However, as described above, the lens units 131 must be designed to match the shapes of the lens units 141, so it is difficult for a lens array set to alter the light projection shape of the LEDs 12. Consequently, in the second optical path, light loss will be caused due to the different aspect ratios of the light projection shape and the shape of the micro-display device 15.
Accordingly, an urgent need exists in the art to provide a solution that can meet demands of both the light availability and the light distribution uniformity by arranging simple optical elements into the limited space of the projection apparatus.